1. Working Principle of Thermocouples
A thermocouple operates based on the Seebeck effect (thermoelectric effect). The core principle is as follows: a closed loop is formed by two different conductors (or semiconductors), A and B. When the two junctions (the hot and cold ends) are at different temperatures, a thermoelectric potential (millivolt-level voltage) is generated in the loop, and the magnitude of this voltage is related to the temperature difference between the two ends.
Hot Junction (Measurement End): Directly in contact with the measured object (such as high-temperature media), sensing the temperature (T) of the object.
Cold Junction (Reference End): Typically at ambient temperature (T), exposed to air or maintained at a constant temperature through a compensation circuit.
The thermoelectric potential formula is:
Where:
The voltage at the hot junction is relative to the reference temperature (usually 0°C).
The voltage at the cold junction is relative to 0°C. By measuring the thermoelectric potential and compensating for the cold junction temperature, the actual temperature of the hot junction can be calculated.
2. Reasons Why Thermocouples Fail to Measure Millivolt Values in Air
When we refer to the “failure to measure millivolt values,” it typically means that the thermoelectric potential is very small (close to zero), rather than there being no signal at all. The main reason for this is that the temperature difference between the hot and cold junctions is nearly zero:
1. Temperature Difference Disappears
The thermocouple generates a thermoelectric potential based on the “temperature difference between the hot and cold ends.” When the thermocouple is placed in air and the hot junction (e.g., the exposed measurement point) is not in contact with a high-temperature object, and if the surrounding air temperature is uniform, the hot and cold junctions will gradually reach the same temperature. In this case, the thermoelectric potential will be too small to measure accurately.
2. Minimal Temperature Fluctuations in the Environment
Even if there are slight temperature differences in the air (e.g., local ventilation), the temperature difference is typically much smaller than the measurement range of the thermocouple. For example, most industrial thermocouples can measure temperature differences as small as 1°C, which corresponds to a thermoelectric potential of only a few microvolts to tens of microvolts. A standard millivoltmeter or measurement module may lack the necessary precision to detect such small values, resulting in the appearance of “no measurable value.”
3. Improper Measurement Setup
If the thermocouple is not correctly forming a closed loop (e.g., loose connections or broken wires), or if the cold junction compensation circuit is faulty, even when a temperature difference exists, the signal may be interrupted, preventing the measurement of the thermoelectric potential. In such cases, the wiring or connection issues should be addressed first.
Conclusion
The core function of a thermocouple is to “measure temperature by generating thermoelectric potential from a temperature difference.” In the air, if the hot and cold junctions have negligible or no temperature difference, the thermoelectric potential becomes almost zero, leading to “no measurable millivolt value.” This is a natural result of how thermocouples work, not a fault. For accurate measurements, ensure that the hot junction is in effective contact with the object being measured (creating a temperature difference) and that the circuit is intact with proper cold junction compensation.